This application claims priority of Japanese patent Application No. 2001-029521, filed on Feb. 6, 2001.
This invention relates to an optical coupling structure to optically couple an optical circuit and an optical fiber, and more specifically to an optical coupling structure to optically couple a single mode plane optical waveguide and a single mode optical fiber.
Recently, wavelength division multiplexing (WDM) optical transmission systems have rapidly developed that are capable of increasing transmission capacity by transmitting a plurality of optical signals each having a different wavelength in a single optical fiber simultaneously.
With the advance of the WDM optical transmission system, there is a big demand for optical devices to multiplex/demultiplex optical signals each having a different wavelength, for instance, a WDM multiplexer/demultiplexer to multiplex or demultiplex two signals each having a different wavelength and an arrayed waveguide grating (AWG) to multiplex or demultiplex optical signals with several tens of wavelengths at a time.
Those optical devices are mainly realized using silica optical waveguides. In an optical transmission system of an intermediate or longer haul, a single mode silica optical fiber which is capable of transmitting optical signals at low loss is used as a transmission line. It is for this reason that both the silica optical waveguide and the silica optical fiber are made of the same quartz materials and therefore can be optically coupled at low loss by direct connection.
Generally, mode field diameters (MFD) of a single mode fiber (SMF) which is widely used for optical transmission systems and a dispersion shifted fiber (DSF) which is also widely used similarly to the single mode fiber are both approximately 9.5xc2x11 xcexcm.
To reduce the coupling loss with such optical fibers, it is considered that a waveguide parameter of the silica optical waveguide should be approximately equal to that of the optical fiber. That is, it is considered preferable if the relative refractive index difference xcex94 between a core and clad of the silica optical waveguide is approximately 0.3%, a shape of the core is a square approximately 8 xcexcm on a side, and the MFD is almost identical to that of the optical fiber, namely approximately 9.5 xcexcm.
On the other hand, the relative refractive index difference xcex94 tends to increase due to the demand for miniaturization. The silica optical waveguide devices such as the WDM multiplexer/demultiplexer and AWG are composed of curved waveguides. The curve loss of the curved waveguide increases as a curvature radius decreases. By increasing the relative refractive index difference xcex94, entrapment of the light intensifies and therefore the curve loss hardly increases even if the curvature radius is reduced. That is, increasing the relative refractive index difference xcex94 can reduce the device area.
When the number of the multiplexed wavelengths becomes more than 32 in an AWG which needs a large area, it is difficult to realize a desired AWG device in a conventional 4-inch diameter substrate even if the relative refractive index difference xcex94 is set to a little larger value of approximately 0.6%. When a waveguide having an even larger relative refractive index difference xcex94 of approximately 1.5%, namely a silica optical waveguide having a high xcex94 is used, it is possible to house an AWG with 64 wavelengths in an approximately 4-inch diameter substrate.
As stated above, a MFD of a silica optical waveguide with a larger relative refractive index difference (is as small as 5 xcexcm and accordingly the coupling loss with a silica optical fiber becomes larger. To solve the above reciprocal relation between the curve loss and the coupling loss, a method has been proposed that reduces the refractive index of a core of a silica waveguide in the coupling part with an optical fiber and at the same time enlarges a core diameter. This method is called a thermally expanded core (TEC) method. In essence, after connecting a silica optical waveguide and a silica optical fiber, a core of the silica optical waveguide near to the connecting part is locally heated with an ultraviolet laser or the like to diffuse the elements doped to the core of the silica optical waveguide. Accordingly, in the area near to the connecting part of the silica optical waveguide, the refractive index of the core reduces and the core diameter enlarges causing the increase of the MFD and the decrease of the coupling loss.
In another method, when patterning of the core of the silica optical waveguide is performed using a photolithography method, the core width of the silica optical waveguide only in the part near to the connecting point is extended so that a MFD of the basic mode approximates to that of the silica optical fiber.
In the TEC method, it is necessary to locally heat at least once per connecting part of the silica optical waveguide and silica optical fiber. In an optical device having a large number of connecting parts like an AWG, the number of the local heating process increases and naturally this is quite troublesome. Furthermore, in this method, once a connecting part is excessively heated, it is impossible to repair the part anymore.
In a conventional system to extend a core width, it is possible to perform a batch forming through patterning of the core. However, the silica optical waveguide becomes a multi mode waveguide in the connecting part, namely the part in which the core diameter is extended and therefore it is unavoidable that a high-order lateral mode generates. The optical signals converted to the high-order mode either cannot or minimally couple with the single mode silica optical fiber and accordingly the coupling efficiency of those optical signals decreases.
It is therefore an object of the present invention to provide an optical connecting structure capable of forming a plurality of connecting parts in one operation and realizing a high coupling efficiency.
Another object of the present invention is to provide an optical coupling structure to optically couple a silica optical waveguide having a high relative refractive index difference xcex94 with a single mode optical fiber at a high efficiency.
An optical coupling structure to connect an optical fiber and a plane optical waveguide according to the invention consists of a core having same cross-sectional dimensions as a core cross sectional dimensions of the plane optical waveguide at one side connecting to the plane optical waveguide and having width and depth smaller than a core diameter of the optical fiber at the other side connecting to the optical fiber and clad to surround the core. At least one of width and depth of the core is tapered along optical axis as near to the other side.
A plane optical circuit to optically connect to optical fiber at a side surface of the plane optical circuit according to the invention consists of a core having a first refractive index, at least one of width and depth of the core being tapered along optical axis in a part near to the side surface, the width and the depth of the core at the side surface being smaller than a core diameter of the optical fiber and clad having second refractive index smaller than the first refractive index to surround the core.
The above configurations make it possible to optically couple the plane optical waveguide and the optical fiber at a high efficiency. It is sufficient as far as at least one of the width and depth of the core is tapered as it approaches to the optical fiber, and therefore it is relatively easy to form the structure. It is especially easy to taper the width of the core as it approaches to the optical fiber. In addition, this structure reduces costs because even if a large number of optical coupling parts are required, it is possible to perform all the tapering in one operation.
These configurations are more effective when the optical waveguide consisting of the core and the clad is a single mode optical waveguide and the optical fiber consists of a single mode optical fiber.
Even if the relative refractive index difference (between the core and clad is larger than that of the optical fiber, the taper configuration functions to approximate both propagation constants so that the optical coupling is performed easier. Consequently, it enables the use of plane optical waveguides having a high hand the ability to make the optical devices utilizing such waveguides more compact.